I'll be honest, while I've worked at companies that were pretty darn good at estimation, I've never worked somewhere where people were able to guess within a 20% error margin how long arbitrary tasks should take. Especially since any given task has a potentially infinite amount of solutions, and solution picks depend on time constraints. If you constrain that time by an extra 20%, it might just be that a weaker solution ends up picked...and without doing the work myself, I can't always easily tell if the right one was picked or not and have to just trust that the person did things right.

Which for an individual totally works and solves the problem. Just trust. Unfortunately people also like fairness in pay and compensation...and that start making things tricky.